Electrodeless fluorescent lamps are disclosed in U.S. Pat. No. 3,500,118 issued Mar. 10, 1970 to Anderson; U.S. Pat. No. 3,987,334 issued Oct. 19, 1976 to Anderson; and Anderson, Illuminating Engineering, April 1969, pages 236 to 244. An electrodeless, inductively-coupled lamp, as disclosed in these references, includes a low pressure mercury/buffer gas discharge in a discharge tube which forms a continuous closed electrical path. The path of the discharge tube goes through the center of one or more toroidal ferrite cores such that the discharge tube becomes the secondary of a transformer. Power is coupled to the discharge by applying a sinusoidal voltage to a few turns of wire wound around the toroidal core that encircles the discharge tube. A current through the primary winding creates a time-varying magnetic flux which induces along the discharge tube a voltage that maintains the discharge. The inner surface of the discharge tube is coated with a phosphor which emits visible light when irradiated by photons emitted by the excited mercury atoms. The lamp parameters described by Anderson produce a lamp which has high core loss and is therefore extremely inefficient. In addition, the Anderson lamp is impractically heavy because of the ferrite material used in the transformer core.
An electrodeless lamp assembly having high efficiency is disclosed in U.S. application Ser. No. 08/624,043 filed Mar. 27, 1996. The disclosed lamp assembly comprises an electrodeless lamp including a closed-loop, tubular lamp envelope enclosing mercury vapor and a buffer gas at a pressure less than about 0.5 torr, a transformer core disposed around the lamp envelope, an input winding disposed on the transformer core and a radio frequency power source coupled to the input winding. The radio frequency source supplies sufficient radio frequency energy to the mercury vapor and the buffer gas to produce in the lamp envelope a discharge having a discharge current equal to or greater than about 2 amperes. The disclosed lamp assembly achieves relatively high lumen output, high efficacy and high axial lumen density simultaneously, thus making it an attractive alternative to conventional VHO fluorescent lamps and high intensity, high pressure discharge lamps.
Electrodeless lamps of the type described above require a closed-loop, tubular lamp envelope. The lamp envelope is hollow and forms a closed-loop but may have a variety of different shapes. The aforementioned U.S. Pat. No. 3,500,118 discloses an oval shaped lamp envelope. A toroidal lamp envelope having a constricted section for ferrite placement is disclosed in the aforementioned U.S. Pat. No. 3,987,334. Japanese Document No. 7-94152 discloses electrodeless lamp envelopes of various shapes, wherein two halves are joined in two places to form a ring.
Insofar as known to applicant, the prior art does not disclose a method for making a closed-loop, tubular lamp envelope that is suitable for automated manufacturing. All known shapes of such closed-loop lamp envelopes are very specific, as is the process for making them. As a result, lamp manufacturing equipment would be expensive and inflexible. It would be impossible to accommodate different glass and lamp sizes, as well as different shapes, utilizing one production line. For example, the aforementioned Japanese Doc. No. 7-94152 discloses the joining of formed glass tubes at abutting tube ends. While this approach is generally feasible in a laboratory environment, the process would be very expensive and impractical in a production environment.